The invention concerns a reflector for X-ray radiation which is curved in a non-circular arc shape along a first cross-section in a plane containing an x-direction (tangential curvature), wherein the reflector is also curved along a second cross-section in a plane perpendicular to the x-direction (sagittal curvature).
An X-ray mirror of this type is disclosed e.g. in DE 44 07 278 A1.
X-ray radiation is electromagnetic radiation as is visible light. Due to the higher energy on the order of keV, the interaction between X-ray radiation and matter is significantly different than with visible light. Considerable difficulties were found in providing effective optical structural elements such as mirrors or lenses for X-ray radiation. The structural elements realized up to now are based mainly on Bragg diffraction and total reflection, both under grazing incidence.
In a flat embodiment, an X-ray mirror on the basis of the Bragg diffraction can only reflect a very small portion of the incident divergent X-ray radiation, since the Bragg condition requires relatively accurate angles of incidence. To solve this problem, curved mirror surfaces and also a locally variable planar separation were suggested. The curvature of the mirror surface and the planar separation may thereby vary along a first direction x which corresponds approximately to the main propagation direction of the X-ray radiation (under grazing incidence). For normal dimensions of X-ray analysis devices, the local radius of curvature is on the order of meters and usually has a parabolic or elliptical shape. It is technically relatively easy to produce. To realize a variable planar separation, a multi-layer mirror design has been used. This type of X-ray mirror is referred to as a “Goebel Mirror” (DE 44 07 278 A1).
The reflectivity of the Goebel mirror is limited in that the divergence of the beam perpendicular to the x-direction in the mirror plane cannot be satisfactorily taken into consideration. Two-dimensional focusing is feasible through a rotationally symmetrical design i.e. a second circular arc-shaped mirror curvature in the plane perpendicular to the x-direction. For typical dimensions of X-ray analysis devices, the mirror must have radii of curvature perpendicular to the x-direction in the millimeter range. It has not been previously possible to produce such a strongly curved X-ray mirror with sufficient accuracy, since sufficiently precise reduction in the surface roughness and waviness of such a strongly curved mirror is difficult. Moreover, it has not been possible up to now to prevent layer thickness errors for multi-layer mirrors in the region of large radii of curvature (i.e. at the mirror edge) using conventional coating techniques (sputtering, molecular beam epitaxy etc.), with a reasonable degree of effort. These coating errors reduce the reflectivity of the X-ray mirrors for the desired X-ray wavelength and introduce scattered rays of other wavelengths.
To still obtain two-dimensional focusing, two one-dimensionally focusing Goebel mirrors, which are rotated relative to each other through approximately 90°, must be used in series. This causes considerable intensity loss.
Another disadvantage of rotationally symmetrical Goebel mirrors is the circular annular beam profile of the reflected X-ray radiation outside of the focus. Either the sample or the detector is usually in the focus and therefore either the detector or the sample must be disposed in the region of the annular beam profile. This reduces the intensity, and the optical path of such an X-ray analysis device lacks flexibility due to the annular beam profile.
Rotationally symmetrical total reflection mirrors with two-dimensional focusing are also known. Due to the reduced light collecting capacity, the very small maximum angle of incidence, the associated adjustment difficulties, and the lack of monochromatization, total reflection mirrors are no practical alternative.
In contrast thereto, it is the object of the present invention to make the design of X-ray mirrors and the beam shape of reflected X-ray radiation more flexible, to facilitate production of X-ray mirrors with high efficiency (i.e. high reflection capacity and good focusing properties).